NOTES ON ACANTHODIAN SHARKS. 
BY 
BASHFORD DEAN. 
From the Zodlogical Laboratory of Columbia University. 
WITH 36 FIGURES. 
The following notes relating to Acanthodian material preserved in 
several European museums were brought together several years ago as 
an incident to another study, but they have now been transcribed for 
publication, since they deal with several unnoted, or little noted, features 
of these ancient sharks. 
The Acanthodians are known to have constituted the earliest line of 
sharks which underwent a period of evolutional prosperity. First known 
from the late Silurian, they gave rise to a series of highly modified forms 
in the Devonian (represented by at least three families and upward of 
twenty species), and became extinct by the close of the Permian. The 
earliest of the remaining groups of sharks, on the other hand, do not 
appear before the Devonian and were indeed sparingly represented during 
the maximum epoch of the Acanthodian ; but by the coal times they had 
evolved characters which, as we have reason to believe, placed them under 
more favorable conditions for survival and thus enabled them to supersede 
their more specialized Acanthodian neighbors. 
Dentition—The dental structures of Acanthodians have received but 
little comment. It was known that while in many genera, Acanthodes, 
Parexus, Diplacanthus, Climatius, and Cheiracanthus, teeth are rudi- - 
mentary (or even lacking), in other forms they are exceedingly con- 
spicuous. Thus, in Acanthodopsis, according to A. 8S. Woodward, there 
are “a few large, laterally compressed, triangular teeth ”; in Ischnacan- 
thus, “a few large conical teeth, the interspaces between these teeth being 
occupied by a close series of minute cusps, all apparently in firm connec- 
tion with a membrane bone in both jaws.” But no details of dentition 
have been given, in spite of the fact that they are of no little importance 
in comparing Acanthodians with other early sharks. Accordingly, we are 
led to give the accompanying figures, 1-11, with comments. 
There are present in Ischnacanthus gracilis, cf. Fig. 1 (from specimen 
AMERICAN JOURNAL OF ANATOMY.—VOL. VII. 
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